1. Field of the Invention
The present invention relates to a fiber assembly using a PANDA (Polarization-maintaining AND Absorbtion-reducing fiber) optical fiber, and more particularly to an optical fiber ferrule assembly equipped with an angular index indicating a polarization plane of an optical fiber.
2. Description of the Related Art
In optical instrumentations and optical communications, there has been known a polarization plane maintaining optical fiber which is capable of carrying out transmission while maintaining its polarization plane. The polarization plane maintaining optical fiber has come into widespread use in coherent communication fields which are affected by the polarization condition or in fields which input and output the characteristics of optical equipment and in other fields which depend upon the optical polarization condition.
The polarization plane maintaining optical fibers can be realized using a double refraction fiber. The polarization plane maintaining optical fiber using the double refraction fiber can structurally be classified into a type which provides a stress and the type in which the core configuration is deformed from a circle to an ellipse. Today, there has been employed a PANDA fiber (Polarization-maintaining AND Absorption-reducing fiber) which is of the stress-providing type. A brief description will be made hereinbelow of the structure of this PANDA fiber with reference to FIG. 9. In FIG. 9, in a cladding 102 of this PANDA fiber 100, there are provided stress-providing members 103A and 103B which are made of a glass having a greater coefficient of thermal expansion than that of a silica glass surrounding them. When the fiber gets cold after being drawn at a high temperature, the shrinkage of the stress-providing members is larger than that of the surrounding material, so that a core 101 is pulled in the Y direction while a compressive stress takes place in the X direction. As a result, owing to the photoelastic effects, the core 101 has different indexes of refraction in the X and Y directions, which makes a difference in a propagation constant of the propagating polarization mode. In addition, although various devices have been studied heretofore on the basis of the methods of providing stresses, today the following two fibers except the PANDA type has been put in practical use. A bow tie type having fan-shaped stress-providing members was developed in the U.K. On the other hand, a jacket (stress-providing member placed around the cladding) type was developed in Japan and has been put in practical use. Further, the types of deforming the core configuration were researched, nevertheless they have not been put in practical use.
For connections between such polarization plane maintaining optical fibers and between the polarization plane maintaining optical fiber and a different device, their polarization planes need to coincide with each other with a high degree of accuracy. For this reason, the connection relies on the individual adjustment. A pair of ferrules which support the optical fibers being coupled are inserted into a sleeve in opposed relation to each other. One of them is coupled to a light source, whereas the other is connected with an optical power meter. In this state, they are fixed when the connection loss is at a minimum. This method is realizable utilizing the structure disclosed in Japanese Unexamined Published Patent Application No. 61-228404.
However, if a precise index indicating the polarization plane for the ferrule for the polarization plane maintaining optical fibers is provided, even if the polarization plane alignment is not done individually, the connections between the ferrules, between the ferrule and the light source and between the ferrule and a device with a polarization characteristic are possible and the individual adjustment becomes unnecessary.